The present invention relates generally to a zoom lens system of three-group construction in which each lens group consists of a single lens alone, and more particularly to a zoom lens system which is suitable for use as an image pickup lens for still cameras, video cameras or the like, and which comprises three groups, each consisting of one lens.
So far, some proposals have been made of a zoom lens system of simple group construction, each group consisting of one single lens. For instance, JP-A 11-6960 discloses a zoom lens optical system of three-group construction, each group consisting of one lens using a gradient index medium rather than a homogeneous medium that is easily available and easy to process.
In view of such a problem with the prior art as mentioned above, it is an object of the present invention to provide a zoom lens system of three-group construction, each consisting of one single lens, which is less susceptible to aberration variations even upon zooming and can achieve entire-length reductions, large lens aperture and high zoom ratio.
According to one aspect of the invention, this object is achievable by the provision of a zoom lens system of three-group construction, which comprises, in order from an object side thereof, a first lens group consisting of one single lens formed of a homogeneous medium and having positive refracting power, a second lens group consisting of one single lens formed of a homogeneous medium and having negative refracting power, and a third lens group consisting of one single lens formed of a homogeneous medium and having positive refracting power, and wherein for zooming from a wide-angle end to a telephoto end of said zoom lens system, at least said second lens group moves from said object side toward an image side of said zoom lens system.
Preferably in this case, the first lens group should consist of one convex lens having a strong convex surface directed to an object plane side of the zoom lens system and the second lens group should consist of one concave lens having a strong concave surface directed toward an image plane side of the zoom lens system.
The zoom lens system of the invention should preferably satisfy the following relation:
fW less than f3 less than fTxe2x80x83xe2x80x83(1)
where fW is a focal length at which the focal length of the zoom lens system is shortest, fT is a focal length at which the focal length of the zoom lens system is longest and f3 is a focal length of the third lens group.
Preferably, a convex lens in the third lens group should have at least one surface defined by an aspherical surface.
Preferably, the convex lens in the third lens group should be a double-convex lens.
Preferably, the zoom lens system of the invention should satisfy the following ratio relation:
xe2x88x923.5 less than f1/f2 less than xe2x88x922.0xe2x80x83xe2x80x83(2)
where f1 is a focal length of the first lens group and f2 is a focal length of the second lens group.
Preferably, the zoom lens system of the invention should satisfy the following ratio relation:
5.0 less than f1/f3 less than 10.0xe2x80x83xe2x80x83(3)
where f1 is a focal length of the first lens group and f3 is a focal length of the third lens group.
Preferably, the zoom lens system of the invention should satisfy the following ratio relation:
0.5 less than L/fT less than 1.5xe2x80x83xe2x80x83(4)
where L is a distance from a rear surface in the second lens group to an image plane of the zoom lens system, at which the focal length of the zoom lens system is longest and fT is a focal length at which the focal length of the zoom lens system is longest.
Preferably, the first lens group should remain fixed during zooming from the wide-angle end to the telephoto end of the zoom lens system.
According to another aspect of the invention, there is provided a display device with a built-in three-group zoom lens system having phototaking function, which comprises:
a first lens group consisting of one positive lens, a second lens group consisting of one negative lens, a third lens group consisting of one positive lens, and an electronic image pickup element for receiving an object image formed by only refracting powers of said first lens group, said second lens group and said third lens group,
a processing circuit for electrically processing information on said object image received by said electronic image pickup element, and
an image display element for displaying the information on said object image fed through said processing circuit to an viewer.
Preferably in this case, the three-group zoom lens system should be constructed in such a way that separations between the first lens group, the second lens group and the third lens group are varied for zooming from a wide-angle end to a telephoto end of the system.
Alternatively, the three-group zoom lens may be constructed for zooming from the wide-angle end to the telephoto end of the system in such a way that the separation between the first lens group and the second lens group becomes wide and the separation between the second lens group and the third lens group becomes narrow.
Preferably, a stop should be located between the second lens group and the third lens group.
A filter may be located between the third lens group and the electronic image pickup element.
The filter may be in a plane-parallel plate form.
The first lens group may consist of one cemented lens comprising a plurality of cemented lens elements.
The first lens group may consist of one lens element.
The second lens group may consist of one cemented lens comprising a plurality of cemented lens elements.
The second lens group may consist of one lens element.
The third lens group may consist of one cemented lens comprising a plurality of cemented lens elements.
The third lens group may consist of one lens element.
The image display element may be built up of a liquid crystal display having a flat screen.
Preferably, the third lens group should have at least one aspherical surface configured to make correction for aberrations.
In the another aspect of the invention, too, the following ratio relation should preferably be satisfied:
fW less than f3 less than fTxe2x80x83xe2x80x83(1)
where fW is a focal length at which the focal length of the zoom lens system is shortest, fT is a focal length at which the focal length of the zoom lens system is longest and f3 is a focal length of the third lens group.
Preferably, the following ratio relation should be satisfied:
xe2x88x923.5 less than f1/f2 less than xe2x88x922.0xe2x80x83xe2x80x83(2)
where f1 is a focal length of the first lens group and f2 is a focal length of the second lens group.
Preferably, the following ratio relation should be satisfied:
5.0 less than f1/f3 less than 10.0xe2x80x83xe2x80x83(3)
where f1 is a focal length of the first lens group and f3 is a focal length of the third lens group.
Preferably, the following ratio relation should be satisfied:
0.5 less than L/fT less than 1.5xe2x80x83xe2x80x83(4)
where L is a distance from a rear surface in the second lens group to an image plane of the zoom lens system, at which the focal length of the zoom lens system is longest and fT is a focal length at which the focal length of the zoom lens system is longest.
The first lens group may remain fixed during zooming from the wide-angle end to the telephoto end.
Preferably, the zoom lens system should be compactly housed in the display device by allowing the first lens group and the second lens group to move in such a way that a distance from a surface located nearest to the object side in the first lens group to the electronic image pickup element is reduced to a length between 20 mm and 2 mm.
Preferably, the first lens group should remain fixed form zooming from the wide-angle end to the telephoto end of the zoom lens system.
Preferably, the separations between the first lens group, the second lens group and the third lens group should be variable in such a way as to satisfy the following zoom ratio condition:
1.2 less than fT/fW less than 25.0xe2x80x83xe2x80x83(6)
where fW is a focal length at which the focal length of the zoom lens system is shortest and fT is a focal length at which the focal length of the zoom lens system is longest.
According to yet another aspect of the invention, there is provided a telephone handset comprising a display device with a built-in two-group zoom lens system having phototaking function, a microphone for entering an operator""s voice therein, a speaker for producing a voice of a person on the other end and a dial button for entering at least a telephone number therein.
The telephone handset may be constructed in the form of a portable telephone set having an antenna for transmitting and/or receiving radio waves.
why the aforesaid arrangements are used and how they word will now be described.
In the zoom lens system of three-group construction according to the present invention, the first lens group consists of one single lens formed of a homogeneous medium and having positive refracting power, the second lens group consists of one single lens formed of a homogeneous medium and having negative refracting power and the third lens group consists of one single lens formed of a homogeneous medium and having positive refracting power, and for zooming the wide-angle end to the telephoto end of the system, at least the second lens group is designed to move from the object side to the image side of the system. Thus, the third lens group takes a chief part in the image-forming action of the lens system. The condition to this end is condition (1). In an intermediate setting, the power of the zoom lens system is substantially equal to the power of the third lens group. As the telephoto or wide-angle end is reached, the field angle varies with the first or second lens group. This arrangement is favorable in view of aberration correction because the height of a ray passing through the third lens group remains substantially unchanged irrespective of zooming settings. For aberration correction, it is favorable to use an aspherical surface in the third lens group because considerable aberrations are produced at the second lens group, and because off-axis coma can be corrected with correction of spherical aberration.
Since, in the invention, the field angle is varied by varying the separation between the first lens group and the second lens group, some refracting power difference must be between both lens groups. Otherwise, the amount of zooming movement increases. When the upper limit of xe2x88x922.0 in condition (2) is exceeded, the refracting power difference between the first lens group and the second lens group vanishes, resulting in an increase in the amount of zooming movement. It is not preferable to be short of the lower limit of xe2x88x923.5 in condition (2) because the refracting powers of the first and second lens groups becomes too large, and aberrations produced there increase accordingly.
The refracting power ratio between the first lens group and the second lens group is defined as mentioned above. However, it is to be understood that too large refracting power causes aberrations to become worse. When the lower limit of 5.0 in condition (3) is not reached, the refracting power of the first lens group becomes too strong, causing aberrations to become worse. When the upper limit of 10.0 in condition (3) is exceeded, on the other hand, the zoom lens system becomes large due to too weak refracting power.
To make the zoom lens system compact, it is required to make short the distance from the rear surface in the second lens group to the image plane. In other words, it is preferable that condition (4) is satisfied while the focal length of the zoom lens system is longest. When the upper limit of 1.5 in condition (4) is exceeded, it is impossible to make the zoom lens system compact. It is not preferable to be short of the lower limit of 0.5 because spherical aberration at the second lens group remains under-corrected.
More preferably, condition (4) should be reduced to:
0.7 less than L/fT less than 1.4xe2x80x83xe2x80x83(5)
Preferably in the zoom lens system of the invention, the separation between the first lens group and the second lens group should be variable in such a way as to satisfy the following zoom ratio (fT/fW) condition:
1.2 less than fT/fW less than 25.0xe2x80x83xe2x80x83(6)
Here fW is a focal length at which the focal length of the zoom lens system is shortest and fT is a focal length at which the focal length of the zoom lens system is longest.
In one preferable embodiment of the invention, this zoom lens system is used with an electronic image pickup element such as a CCD to set up a display device having phototaking function for an electronic camera or the like. According to this embodiment, the zoom lens system can be compactly housed in the display device by allowing the first and second lens groups to move in such a way that the distance from the surface located nearest to the object side in the first lens group to the electronic image pickup element is reduced to the length between 20 mm and 2 mm.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.